You can convert your DRM-free ebooks for a Kindle using Calibre (has its own forum here). If they have DRM (encryption), as many commercial ebooks from major publishers do, then you would first have to remove the DRM and then use Calibre. See
Adobe ADEPT DRM for EPUB circumvented.
If you want wireless access to stores and to keep your existing ePub ebooks "as is", then your best bet is the BeBook Neo (or the identical Onyx Boox 60). The PocketBook 302 may also be an option, but it is brand new. Both come with WiFi, rather than 3G.